1. Field of the Invention
The present invention relates to a method of preparing a volume type phase hologram film, and more particularly to a method of preparing a volume type phase hologram film having sufficient flexibility and further a superiority in high diffraction efficiency, high transparency and high moisture resistance, the method being capable of being used for various purposes.
2. Related Background Art
Holography is a unique technique of forming an optical image by which an object is irradiated by an appropriately coherent wave, such as a laser beam, where the wave is modulated in amplitude and phase in accordance with the shape of the object; the modulated wave reflected upon or transmitted through the object is recorded (=hologram) and the hologram is irradiated again by the laser beam so as to reproduce an optical image of the original object.
With the recent development of holography, it has been clarified to a certain extent what material is suitable for use in holography or what characteristics, such as a hologram recording material, should have.
Thus, there have already been proposed various hologram materials such as bleached silver salt (U.S. Pat. No. 3,672,744), dichromate gelatine (U.S. Pat. No. 3,617,274), photoresists, thermoplastic resin, inorganic glass materials and ferroelectric substances. At present further studies on the properties of these materials is proceeding in the art.
The properties which a hologram recording material (or sensitive material) should have may be summarized as follows:
(1) sensitivity to a laser beam, especially to a laser beam in the visible wavelength region and high sensitivity at the same time; PA0 (2) high resolving power; PA0 (3) to form a hologram of high diffraction efficiency; PA0 (4) to provide a hologram of low noise level; PA0 (5) to yield a stable hologram; and PA0 (6) to allow easy recording and reproducing operations.
As will be seen from the above, the requirements for a hologram recording material are very severe.
Practically, very few known hologram recording materials can satisfy the above requirements completely or at least partially to the extent that their use may be practical.
Among the above-mentioned materials, bleached silver salt and dichromate gelatine may be considered to be usable practically. However, they have particular disadvantages. The former necessitates a bleaching treatment in addition to ordinary treatment and furthermore the hologram obtained from it is poor in light fastness. The latter has a problem regarding the preservation of the hologram because the hologram obtained from this material lacks adequate stability against moisture.
To overcome such disadvantages in conventional holograms, U.S. Pat. No. 4,287,277 discloses a hologram employing polyvinyl carbazole. A hologram employing polyvinyl carbazole is suitable in use as it can satisfy the properties (1) to (6) mentioned above.
In general, volume type phase holograms are holograms in which a Bragg's grating is formed in the inside of a polymer. Diffraction light wavelength is controlled by the grating distance (or grating space), where the grating pattern comprising several thousand or more lines, is formed in a thickness of several ten .mu.m with a grating space of 0.1 to 0.2 .mu.m. Therefore, at the time of manufacturing a volume type phase hologram (particularly at the time of exposure to laser beam), the exposure to a laser beam must be carried out in a stationary state so that fluctuation may be suppressed within approximately 0.01 to 0.02 .mu.m.
More specifically, at the time of the exposure to light, vibration, air-swirling caused by the passage of a laser beam, deformation of a holder of a sensitive material, thermal expansion, density gradient owing to temperature of air, etc. must be made insignificantly small. For that purpose a rigid and transparent material is used as a support for supporting the sensitive material, and, in general, glass is usually used.
However, the utility of holograms for which glass is used as the support is of very limited scope, such as displays or the like.